1. Field of the Invention
This invention relates to a compound and a process for sealing spaces in electrical instruments.
2. Description of the Prior Art
It is well known in the art to seal spaces in high voltage electrical instruments with various liquid or solid dielectric substances in order to reduce the possibility of short circuiting. However, such known substances provide substantial disadvantages which are inherent in their use. Thus, for example, if insulating oil is used as the dielectric substance, it is necessary to equip the instrument with an oil reservoir to compensate for the thermal expansion and contraction caused by both environmental and internal conditions. Moreover, the insulating oil is subject to leakage from the instrument, particularly in those instances wherein the instrument must be transported to its final site.
In an effort to remedy the problems caused by liquid dielectric substances, the art has turned to substances which, although initially introduced as a liquid, harden to a solid dielectric substance. In most instances, however, it has been found that the hardening process and/or thermal conditions within the instrument cause cracking of the solid which naturally results in degradation of the dielectric properties. Additionally, the solid consistency of the substance makes internal repair work of the instrument an extremely difficult task. In this regard, despite the use of special solvents, such as dimethylformamide, a great amount of time and energy must still be expended to accomplish the repair.
Published German patent application No. 20 50 232 describes a variation of the solid insulating material. Liquid rubber is poured into the instrument and vulcanized therein by heating the instrument. Although providing the proper insulative qualities, the relatively incompressable material exerts large pressures on the internal mechanism of the instrument upon the occurrence of higher temperatures due to its large thermal expansion coefficient. Thus, the material can only be effectively used in those situations where a thin layer of material is required or relatively small increases in temperature are experienced.
A further known technique for sealing high voltage electrical instruments involves the use of gel-like materials. Such materials have been prepared, for example, by adding thickeners, represented by silicic acid* .sup.), to the previously mentioned insulating oils. To obtain a degree of compressability, microballoons may be admixed with the thickened oil. A problem associated with this material, however, is that unless carefully controlled, the viscosity of the material is such that it is difficult to fill all the open spaces of the instrument. As can readily be appreciated, the resulting gaps adversely affect the dielectric properties of the insulating material. FNT *.sup.) correct is: pyrogenic silica
A further example of an insulating gel-like material is disclosed in Swiss Pat. No. 526,842. The described material is prepared by adding powdery polystyrene to a chlorinated diphenyl compound at ambient temperature, introducing the mixture to the instrument and heating the entire instrument to gel the material. To impart compressability to the material a gaseous substance may be incorporated prior to introducing the material into the instrument. Although this material possesses the proper dielectric properties and is beneficially non-combustible, it requires the heating step to gel and in the event of prolonged exposure to elevated temperatures (i.e., above 90.degree. C.) tends to liquify.
From the above description of the prior art, it should be apparent that the art has searched for a sealing compound which is relatively simple to prepare and introduce into the electrical instrument, but remains as an effective dielectric substance over a broad range of operating conditions. This invention was developed as a result of that search.